RELATING MATERIAL PROPERTIES AND HYDRAULIC CONDUCTIVITY OF TERTIARY VOLCANIC-ROCK UNITS IN THE SOUTHWESTERN NEVADA VOLCANIC FIELD, NEVADA

Plausible simulations of ground-water flow require a geologically consistent framework that is quantitatively related to the hydraulic properties of the simulated hydrogeologic units. However, it is difficult to accurately quantify the geologic and hydraulic properties at regional scales because rock properties can vary greatly in space and hydraulic data tends to be defined at points. Tertiary volcanic units of the southwestern Nevada volcanic field (SWNVF) form a series of aquifers and confining units that are being modeled at local and regional scales to evaluate potential transport of radionuclides from the Nevada Test Site and the Yucca Mountain site. These volcanic units have spatially varying material properties produced by lithology, welding, fracturing, alteration, and proximity to inferred source. Geologic and hydraulic conductivity estimates were compiled for 6 major units of the SWNVF from boreholes where hydraulic tests were done. Hydraulic conductivity was linearly regressed with estimated percentages of alteration and welding. Alteration could not be related to hydraulic conductivity, but a correlation may exist between welding and hydraulic conductivity. Quantitatively predicting hydraulic conductivity based on either alteration or welding is not possible given the current data. However, statistical hypothesis testing qualitatively supports that hydraulic conductivity increases with increased welding due to additional fracturing and hydraulic conductivity decreases with increased alteration due to occluded porosity when classified along nominal scales of alteration and welding. These qualitative relationships allow contour maps of percentages of alteration and welding to be used in defining zones of differing hydraulic conductivity. Correspondence between rock properties and hydraulic conductivity was best where lithologic variability was limited (e.g., lava flows and nonwelded tuffs) and poorest in thick, complex welded tuffs. Poor correspondence between rock properties and hydraulic properties may be from variable connectivity of fractures encountered during a hydraulic test. Zone maps may be appropriate at a regional scale, but these results may not be readily applicable to site-scale models where knowledge of a specific flow path is required.